Part IV on cams concludes Valve Springs
In 1999, when installing high-performance cams and mating heavier seat-pressure valve springs in the Twin Cam, leading performance mechanics discovered that all the expected performance was not forthcoming. The reason for installing heavier seat pressure valve springs in an engine is to ensure that the valvetrain precisely follows the instructions laid out by the cam lobe profile. In other words, the valves open and close when instructed to by the turning elliptical cam lobe. The performance valve springs’ mandate is to prevent valve float and valve bounce, which rob horsepower and can destroy an engine.
Valve float occurs when an errant valve doesn’t return to its seat. The valve hangs around in the combustion chamber after the cam lobe has rotated to a lower lift, allowing the valve springs to begin closing the valve onto its mating seat. High rpm or wild cam profiles with all lift and no duration can easily confuse the valve as the properties of momentum and inertia come into play. At the peak of the cam lobe, the valve will want to keep going (momentum) to open farther under these conditions. It may open farther or oscillate (inertia) in no man’s land until the valve springs take charge and force the errant valve back to its valve seat. Valve bounce, the lesser-known cousin of valve float, occurs when the valve hits its mating seat at high rpm. The valve tends to bounce off the seat, causing much the same problems as float. Heavier seat valve springs will correct this condition, also. Valve spring pressure should be closely matched to an engine’s potential rpm range and the profile of the cam. It’s a good idea to use the stock valve springs with stock or mild-lift performance cams.
However, the stock cam timing chains and adjusters, particularly those used in 1999-2006, should be replaced with a pure gear drive produced by the likes of Andrews, Crane, S&S, or Wood Performance. Bobby Wood introduces the exotic leading-edge combination of an outboard, carbon-fiber belt with either the spur or helical gear inboard gear drives of the Knight Prowler advanced silent belt drive system when using higher lift cams requiring heavier seat-pressure valve springs. I agree with S&S Cycle that stock timing chains and their mating chain tensioners are not strong enough to counter the extra seat pressure of performance valve springs.
In these situations, cam timing can vary by 4 degrees, and this is a direct result of the loose fit of the stock cam sprockets. One only has to look at these sprockets to see that exact timing is impossible. Sloppy parts lead to sloppy timing. Therefore, the gear setup can account for more ponies because timing will be much more exact. Furthermore, if the chain tensioners are not strong enough to dictate the tension of the timing chains, then cam timing can and will vary beyond the 4 degrees mentioned above. How far it varies depends on the seat pressure of the valve springs, the lobe profile of the cams, and the rpm range. This is not good. This more than accounts for the missing horsepower expected back in 1999 performanced Twin Cams. There are other potential problems that will occur, as time goes on, with both stock and high-performance engines using stock gearcase components. Stock TC 88 chain adjusters can wear with resultant oil supply contamination problems, along with wear of the timing chains themselves.
Gear-driven camshafts save more horsepower through the elimination of chain whip, lash, or flex, whatever term you choose to use. Additionally, more horsepower gained by the use of proper performance valve springs expands the use of other coordinated support parts, like performance camshafts. The aftermarket gear drives are an improvement in reliability and performance.
Beehive Valve Springs
In 1999, the Twin Cam used the Evo valve springs instituted in 1984. These dual cylindrical valve springs (#18201-83 outer and #18202-83 inner), one inside the other, hold each valve closed on early Twin Cams up until early 2004. The performance version of these valve springs has been around since late 1984. They are made in a variety of materials from functional steel to lighter and stronger exotic materials, such as titanium and processed nitriding. There is a wide choice of seat pressures providing matches to all cam profiles. There is just as wide a choice in manufacturers. The inner spring is normally counter wound to the outer to combat fibrillation. The different sizes of inner and outer also combat this untoward spring movement away from designed compression and extension under duress of performance cam design. The springs may also be progressively wound, whereby coils are closer together at one end than the other to prevent harmonics from impeding intended travel and function. A single beehive-shaped valve spring (#18245-02), operates the closing of each valve on later Twin Cams from circa-2004 on.
The valves also have narrower diameter valve stems, which reduce valve, and thus valvetrain, weight. Any time valvetrain weight reduces, two beneficial results emerge: the engine can rev higher, since there is less mass to move up and down, and a lower seat pressure will perform the intended functions, since there is less weight to impede the valve’s return to its seat. Less weight combined with an effective lower seat-pressure spring reduces valve seat slam from the returning valve. The stock beehive spring seat pressure is only 130 pounds with the valve on its mating valve seat. Compression steadily increases seat pressure. Common dual seat pressures for Crane springs, as an example, are 155 pounds or 175 pounds for more demanding cam profiles. Interestingly, Screamin’ Eagle heads for circa-2005 and later applications, which normally use the single beehive spring, substitute dual cylindrical springs with seat pressures and clearance to accommodate up to .650" lift cams (stock lift is .473"). It looks to me like SE may not like its stock counterpart spring design.
Bob Wood proudly proclaims, “Wood Performance has stepped up to the plate to give our customers the absolute latest and best-kept secrets in NASCAR valvetrain technology, and incorporated it into our latest W160-PSW-7L Knight Prowler valve springs.” The valves using stock beehive springs rotate inside their valve key locks. Valves have indents machined into them just below the top of the valve stem. The tapered valve locks, two per valve, are inserted into the mating valve stem indent with the valve spring compressed. The top valve collar and spring are slowly released until the mating taper in the top valve collar locks onto the keys, which are rounded. Each circumnavigates almost 180 degrees around the stem to complete the circle. The Knight Prowler springs feature titanium 7-degree, 7mm posi-lock valve keepers and retainers. They have 160 pounds of seat pressure at 2" compression, 360 pounds at 1.400" compression, 390 pounds at 1.300" compression, and coil bind at 1.230" compression. Bob’s valve springs accommodate up to .600" lift and still maintain a safe interference clearance of 0.060" without shimming or cutting the valve guides. They feature an advanced 1200 series, heat-treating process with an additional 1500 series nitriding.
Conclusion
Next month, we’ll continue talking about cam characteristics, as well as take a close look at Bob Wood’s Knight Prowler belt drive cam setup.
Donny Petersen
Tattoo Tony’s Heavy Duty Cycles
Toronto, Canada
www.HeavyDutyCycles.com 
Techline-August08-PtIV Cam Specs and Choices (conclusion) 
Linear Mode
